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Generate branchless code when "if" can be evaluated at compile time

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We already avoid the conditional branch, but still have an
unconditional branch in the generated code. Remove it.
This commit is contained in:
Björn Steinbrink 2013-07-25 17:03:46 +02:00
parent e0685e22a3
commit 7078ab7324
1 changed files with 14 additions and 18 deletions
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32
src/librustc/middle/trans/controlflow.rs
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@ -74,20 +74,18 @@ pub fn trans_if(bcx: @mut Block,
if is_const(cond_val) && !is_undef(cond_val) {
if const_to_uint(cond_val) == 1 {
// if true { .. } [else { .. }]
let then_bcx_in = scope_block(bcx, thn.info(), "if_true_then");
let then_bcx_out = trans_block(then_bcx_in, thn, dest);
let then_bcx_out = trans_block_cleanups(then_bcx_out,
block_cleanups(then_bcx_in));
Br(bcx, then_bcx_in.llbb);
return then_bcx_out;
return do with_scope(bcx, thn.info(), "if_true_then") |bcx| {
let bcx_out = trans_block(bcx, thn, dest);
trans_block_cleanups(bcx_out, block_cleanups(bcx))
}
} else {
match els {
// if false { .. } else { .. }
Some(elexpr) => {
let (else_bcx_in, else_bcx_out) =
trans_if_else(bcx, elexpr, dest, "if_false_else");
Br(bcx, else_bcx_in.llbb);
return else_bcx_out;
return do with_scope(bcx, elexpr.info(), "if_false_then") |bcx| {
let bcx_out = trans_if_else(bcx, elexpr, dest);
trans_block_cleanups(bcx_out, block_cleanups(bcx))
}
}
// if false { .. }
None => return bcx,
@ -107,7 +105,8 @@ pub fn trans_if(bcx: @mut Block,
// 'else' context
let (else_bcx_in, next_bcx) = match els {
Some(elexpr) => {
let (else_bcx_in, else_bcx_out) = trans_if_else(bcx, elexpr, dest, "else");
let else_bcx_in = scope_block(bcx, elexpr.info(), "else");
let else_bcx_out = trans_if_else(else_bcx_in, elexpr, dest);
(else_bcx_in, join_blocks(bcx, [then_bcx_out, else_bcx_out]))
}
_ => {
@ -125,9 +124,8 @@ pub fn trans_if(bcx: @mut Block,
return next_bcx;
// trans `else [ if { .. } ... | { .. } ]`
fn trans_if_else(bcx: @mut Block, elexpr: @ast::expr,
dest: expr::Dest, scope_name: &str) -> (@mut Block, @mut Block) {
let else_bcx_in = scope_block(bcx, elexpr.info(), scope_name);
fn trans_if_else(else_bcx_in: @mut Block, elexpr: @ast::expr,
dest: expr::Dest) -> @mut Block {
let else_bcx_out = match elexpr.node {
ast::expr_if(_, _, _) => {
let elseif_blk = ast_util::block_from_expr(elexpr);
@ -137,11 +135,9 @@ pub fn trans_if(bcx: @mut Block,
trans_block(else_bcx_in, blk, dest)
}
// would be nice to have a constraint on ifs
_ => bcx.tcx().sess.bug("strange alternative in if")
_ => else_bcx_in.tcx().sess.bug("strange alternative in if")
};
let else_bcx_out = trans_block_cleanups(else_bcx_out,
block_cleanups(else_bcx_in));
(else_bcx_in, else_bcx_out)
trans_block_cleanups(else_bcx_out, block_cleanups(else_bcx_in))
}
}